This invention relates to optical fiber communication cables including a large number of optical fibers.
Optical fiber cables are used for transmitting information between various devices including telephone exchanges, computers, etc. Although each fiber can transmit relatively large amounts of information by way of light signals guided by the fibers, there is a demand for cables with a large number of fibers, e.g. 145 to 1250 or larger. Presently installed, or available, cables have fewer fibers.
The optical fibers are small in diameter, a few microns, and are relatively fragile. Therefore, they must be protected from mechanical stresses encountered during manufacture, installation and operation such as bending and pulling stresses. One known way of protecting optical fibers is to place them loosely in tubes with the fibers longer than the tubes and to associate with such tube a tensile member, e.g. a stranded metal wire, a high strength plastic or carbon, glass or graphite fibers, which absorbs the tensile stresses. The tubes with the tensile member are enclosed in a sheath or sheaths. See, for example, U.S. Pat. Nos. 4,078,853; 4,153,332; and 4,230,395.
With the optical fiber loosely received in the tubes, there are unfilled spaces within the tubes, and the fibers should be protected from moisture and other hydrogen compounds. It is known to fill such tubes with a grease-like material which prevents migration of such moisture and compounds in the tubes but which permits relative movement of the fibers and their enclosing tubes. See, for example, U.S. Pat. Nos. 4,230,395; 4,722,589 and 4,725,121.
When the tubes containing the fibers are of circular cross-section and are encircled by a sheath of circular cross-section, there are spaces between the tubes, and it is desirable to prevent the ingress of moisture into the latter spaces. It is known to fill such latter spaces with a grease-like material similar to, or the same as, the grease-like material used to fill the tubes, but which permits relative movement of the tubes. See, for example, U.S. Pat. No. 4,230,395.
Such tubes containing the optical fibers or the optical fibers themselves are often wound helically around a central supporting structure which can contain the tensile member. In some cases, they are placed on the central supporting structure in what is called an S or Z shaped configuration, or in other words, they are wound around the central supporting structure in a first direction, or hand, for one or several turns and then, the direction of winding is reversed for one or several turns. Such reversal is continued periodically. See, for example, U.S. Pat. Nos. 4,697,875; 4,722,589 and 4,725,121.
Ribbons of plastic material encasing several optical fibers in side-by-side relation, also known as "parallel" fibers, are known in the art and have several advantages including protection of the optical fibers individually, ease of handling and ease of identification of individual fibers. See, for example, U.S. Pat. Nos. 3,883,218; 3,937,559; 4,078,853 and 4,185,887. Such ribbons have been wound helically around a central supporting member and have been inserted loosely in tubes wound around a centrally disposed one of such tubes.
However, it has been assumed that by encasing the optical fibers in a ribbon of plastic, the fibers have adequate moisture protection without the need for a moisture migration preventing, or hydrogen absorbing, material.
Because of their advantages, such ribbons of plastic material encasing a plurality of optical fibers are especially suitable for interconnecting information transmitting and receiving terminals. Typically, cables containing up to 144 optical fibers have been used, and to meet further traffic demand, separate parallel cables have been installed. A demand has arisen for an optical fiber cable containing more than 144 optical fibers, i.e. in excess of 144 optical fibers and up to at least 1250 optical fibers. Furthermore, it is not sufficient that connections be made only at the ends of such a cable which, at this stage of development, can have a length of tens of kilometers without interruption. Instead, particularly when the cable has 145 or more optical fibers, connections to fibers of the cable intermediate its length are needed. It is highly desirable to be able to connect a number of fibers in the cable to a station intermediate the ends of the cable without disturbing others of the fibers in the cable. If the optical fibers are helically wound around the core or are in tubes helically wound around the core, it becomes difficult to connect to the optical fibers at an intermediate point of the cable because there is insufficient slack in the optical fibers or tubes, and the slack which is available is needed to prevent damage to the fibers during handling, bending and temperature changes, optical fibers being notoriously fragile. Furthermore, access to inner fibers is hampered by outer fibers.
It has been proposed that cables with helically wound fibers be interconnected by an adapter with an S or Z winding of the optical fibers. See application Ser. No. 671,537, filed Mar. 19, 1991 and entitled "Joint For Optical Cables And Method Of Making Such Joint". However, such proposal requires that the cable be severed transversely to its axis, a special adapter be provided and that a number of optical fibers, equal to the number of fibers in each cable be included in the adapter. Then, a number of fiber interconnections which is double the number of fibers to be interconnected is made. Such proposed interconnection is impractical with a large number of optical fibers.
Also, when a cable having a plurality of optical fiber ribbon units does not have a moisture or hydrogen absorbing compound therein, which the prior art has considered to be unnecessary, the optical fibers are subject to moisture, and consequent damage, when the cable is opened for a branch connection to one or more of the ribbon units.